INTERNET-DRAFT Erik Nordmark
Sept 22, 2004 Sun Microsystems
Tony Li
Threats relating to IPv6 multihoming solutions
<draft-ietf-multi6-multihoming-threats-01.txt>
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Abstract
This document lists security threats related to IPv6 multihoming.
Multihoming can introduce new opportunities to redirect packets to
different, unintended IP addresses.
The intent is to look at how IPv6 multihoming solutions might make
the Internet less secure than the current Internet, without studying
any proposed solution but instead looking at threats that are
inherent in the problem itself. The threats in this document build
upon the threats discovered and discussed as part of the Mobile IPv6
work.
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INTERNET-DRAFT Threats to IPv6 multihoming solutions Sept 22, 20041. INTRODUCTION
The goal of the IPv6 multihoming work is to allow a site to take
advantage of multiple attachments to the global Internet without
having a specific entry for the site visible in the global routing
table. Specifically, a solution should allow hosts to use multiple
attachments in parallel, or to switch between these attachment points
dynamically in the case of failures, without an impact on the
transport and application layer protocols.
At the highest level the concerns about allowing such "rehoming" of
packet flows can be called "redirection attacks"; the ability to
cause packets to be sent to a place that isn't tied to the transport
and/or application layer protocol's notion of the peer. These
attacks pose threats against confidentiality, integrity, and
availability. That is, an attacker might learn the contents of a
particular flow by redirecting it to a location where the attacker
has a packet recorder. If, instead of a recorder, the attacker
changes the packets and then forwards them to the ultimate
destination, the integrity of the data stream would be compromised.
Finally, the attacker can simply use the redirection of a flow as a
denial of service attack.
This document has been developed while considering multihoming
solutions architected around a separation of network identity and
network location, whether or not this separation implies the
introduction of a new and separate identifier name space. However,
this separation is not a requirement for all threats, so this
taxonomy may also apply to other approaches. This document is not
intended to examine any single proposed solution. Rather, it is
intended as an aid to discussion and evaluation of proposed
solutions. By cataloging known threats, we can help to ensure that
all proposals deal with all of the available threats.
1.1. Assumptions
This threat analysis doesn't assume that security has been applied
other security relevant parts of the Internet, such as DNS and
routing protocols, but it does assume that at some point in time at
least parts of the Internet will be operating with security for such
key infrastructure. With that assumption it then becomes important
that a multihoming solution would not, at that point in time, become
the weakest link. This is the case even if, for instance, insecure
DNS might be the weakest link today.
This document doesn't assume that the application protocols are
protected by strong security today or in the future. However, it is
still useful to assume that the application protocols which care
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about integrity and/or confidentiality apply the relevant end-to-end
security measures, such as IPsec, TLS, and/or application layer
security.
For simplicity we assume that an on-path attacker can see packets,
modify packets and send them out, and block packets from being
delivered. This is a simplification because there might exist ways,
for instance monitoring capability in switches, which allow
authenticated and authorized users to observe packets without being
able to send or block the packets.
[DISCUSSION: It isn't clear to what extent the above simplifying
assumption is misleading, that is, whether we need to make a
distinction between on-path attackers which can monitor packets
and perhaps also inject packets, without being able to block
packets from passing through.
On-path attackers that only need to monitor might be lucky and
find a non-switched Ethernet in the path, or use capacitive or
inductive coupling to listen on a copper wire. But if the
attacker is on an Ethernet that is on the path, whether switched
or not, the attacker can also employ ARP/ND spoofing to get access
to the packet flow which allows blocking as well. Similarly, if
the attacker has access to the wire, the attacker can also place a
device on the wire to block. Other on-path attacks would be those
that gain control of a router or a switch (or gain control of one
of the endpoints) and I think those would allow blocking as well.
So the strongest currently known case where monitoring (and
injecting?) is easier than blocking, is when switches and routers
have monitoring capabilities (for network management or for lawful
intercept) where an attacker might be able to bypass the
authentication and authorization checks for those capabilities.]
We assume that an off-path attacker can neither see packets between
the peers (for which it is not on the path) nor block them from being
delivered. Off-path attackers can in general send packets with
arbitrary IP source addresses and content, but such packets might be
blocked if ingress filtering [INGRESS] is applied. Thus it is
important to look at the multihoming impact on security both in the
presence and absence of ingress filtering.
1.2. Authentication, Authorization, and Identifier Ownership
The overall problem domain can be described using different
terminology.
One way to describe it is that it is necessary to first authenticate
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the peer and then verify that the peer is authorized to control the
locators used for a particular identifier. While this is correct, it
might place too much emphasis on the authorization aspect. In this
case the authorization is conceptually very simple; each host (each
identifier) is authorized to control which locators are used for
itself.
Hence there is a different way to describe the same thing. If the
peer can somehow prove that it is the owner of the identifier, then
the peer can control the locators that are used with the identifier.
This way to describe the problem is used in [OWNER].
Both ways to look at the problem, hence both sets of terminology, are
useful when trying to understand the problem space and the threats.
2. TERMINOLOGY
link - a communication facility or medium over which nodes
can communicate at the link layer, i.e., the layer
immediately below IPv6. Examples are Ethernets
(simple or bridged); PPP links; X.25, Frame Relay,
or ATM networks; and Internet (or higher) layer
"tunnels", such as tunnels over IPv4 or IPv6 itself.
interface - a node's attachment to a link.
address - an IP layer name that has both topological
significance (i.e., a locator) and identifies an
interface. There may be multiple addresses per
interface.
locator - an IP layer topological name for an interface or a
set of interfaces. There may be multiple locators
per interface.
identifier - an IP layer identifier for an IP layer endpoint
(stack name in [NSRG]), that is, something that
might be commonly referred to as a "host". The
transport endpoint name is a function of the
transport protocol and would typically include the
IP identifier plus a port number. There might be
use for having multiple interfaces per stack/per
host.
The identifiers are not associated with an
interface.
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address field
- the source and destination address fields in the
IPv6 header. As IPv6 is currently specified this
fields carry "addresses". If identifiers and
locators are separated these fields will contain
locators.
FQDN - Fully Qualified Domain Name [FYI18]
3. TODAY'S ASSUMPTIONS AND ATTACKS
The two interesting aspects of security for multihoming solutions are
the assumptions made by the transport layer and application layer
protocols about the identifiers that they see on one hand, and the
existing abilities to perform today various attacks related to the
identity/location relationship, on the other hand.
3.1. Application Assumptions
In the Internet today, the initiating part of applications either
starts with a FQDN, which it looks up in the DNS, or already has an
IP address from somewhere. For the FQDN to IP address lookup the
application effectively places trust in the DNS. Once it has the IP
address, the application places trust in the routing system
delivering packets to that address. Applications that use security
mechanisms, such as IPsec or TLS, with mutual authentication have the
ability to "bind" the FQDN to the cryptographic keying material, thus
compromising the DNS and/or the routing system can at worst cause the
packets to be dropped or delivered to an entity which does not posses
the keying material.
At the responding (non-initiating) end of communication today, we
find applications that fall into approximately five classes with
respect to their security requirements.
The first class is the set of public content servers. These systems
provide data to any and all systems and are not particularly
concerned with confidentiality, as they make their content available
to all. However, they are interested in data integrity and denial of
service attacks. Having someone manipulate the results of a search
engine, for example, or prevent certain systems from reaching a
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search engine would be a serious security issue.
The second class of applications use existing IP source addresses
from outside of their immediate local site as a means of
authentication without any form of verification. Today, with source
IP address spoofing and TCP sequence number guessing as rampant
attacks [RFC1948], such applications are effectively opening
themselves for public connectivity and are reliant on other systems,
such as firewalls, for overall security. We do not consider this
class of systems in this document because they are in any case fully
open to all forms of network layer spoofing.
The third class of applications receive existing IP source addresses,
but attempt some verification using the DNS, effectively using the
FQDN for access control. (This is typically done by performing a
reverse lookup from the IP address followed by a forward lookup and
verifying that the IP address matches one of the addresses returned
from the forward lookup.) These applications are already subject to
a number of attacks using techniques like source address spoofing and
TCP sequence number guessing since an attacker, knowing this is the
case, can simply create a DoS attack using a forged source address
that has authentic DNS records. In general this class of
applications is strongly discouraged, but it is probably important
that a multihoming solution doesn't introduce any new and easier ways
to perform such attacks. However, we note that some people think we
should treat this class the same as the second class of applications.
The fourth class of applications use cryptographic security
techniques to provide both a strong identity for the peer and data
integrity with or without confidentiality. Such systems are still
potentially vulnerable to denial of service attacks that could be
introduced by a multihoming solution.
Finally, the fifth class of applications use cryptographic security
techniques but without strong identity (such as opportunistic IPsec).
Thus data integrity with or without confidentiality is provided when
communicating with an unknown/unauthenticated principal. Just like
the first category above such applications can't perform access
control based on network layer information since they do not know the
identity of the peer. However, they might perform access control
using higher-level notions of identity. The availability of IPsec
(and similar solutions) together with channel bindings allow
protocols which in themselves are vulnerable to MiTM-attacks to
operate with a high level of confidentiality in the security of the
identification of the peer. A typical example is the Remote Desktop
Protocol (RDP) which when used with opportunistic IPsec works well if
channel bindings are available. Channel bindings provide a link
between the IP-layer identification and the application protocol
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identification. This is an important aspect of security in
application protocols which must be preserved by a multihoming
solution.
The requirement for a multihoming solution is that security be no
worse than it is today in all situations. Thus, mechanisms that
provide confidentiality, integrity, or authentication today should
continue to provide these properties in a multihomed environment.
3.2. Redirection Attacks Today
This section enumerates some of the redirection attacks that are
possible in today's Internet.
If routing can be compromised, packets for any destination can be
redirected to any location. This can be done by injecting a long
prefix into global routing, thereby causing the longest match
algorithm to deliver packets to the attacker.
Similarly, if DNS can be compromised, and a change can be made to an
advertised resource record to advertise a different IP address for a
hostname, effectively taking over that hostname. More detailed
information about threats relating to DNS are in [DNS-THREATS].
Any system that is along the path from the source to the destination
host can be compromised and used to redirect traffic. Systems may be
added to the best path to accomplish this. Further, even systems
that are on multi-access links that do not provide security can also
be used to redirect traffic off of the normal path. For example, ARP
and ND spoofing can be used to attract all traffic for the legitimate
next hop across an Ethernet. And since the vast majority of
applications rely on DNS lookups, if DNSsec is not deployed, then
attackers that are on the path between the host and the DNS servers
can also cause redirection by modifying the responses from the DNS
servers.
In general the above attacks work only when the attacker is on the
path at the time it is performing the attack. However, in some cases
it is possible for an attacker to create a DoS attack which remains
at least some time after the attacker has moved off the path. An
example of this is an attacker which uses ARP or ND spoofing while on
path to either insert itself or send packets to a black hole (a non-
existent L2 address). After the attacker moves away the ARP/ND
entries will remain in the caches in the neighboring nodes for some
amount of time; a minute or so in the case of ARP. This will result
in packets continuing to be black-holed until ARP entry is flushed.
Finally, the hosts themselves that terminate the connection can also
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be compromised and can perform functions that were not intended by
the end user.
All of the above protocol attacks are the subject of ongoing work to
secure them (DNSsec, security for BGP, Secure ND) and are not
considered further within this document. The goal for a multihoming
solution is not to solve these attacks. Rather, it is to avoid
adding to this list of attacks.
3.3. Packet Injection Attacks Today
In today's Internet the transport layer protocols, such as TCP and
SCTP, which use IP, use the IP addresses as the identifiers for the
communication. In the absence of ingress filtering [INGRESS] the IP
layer allows the sender to use an arbitrary source address, thus the
transport protocols and/or applications need some protection against
malicious senders injecting bogus packets into the packet stream
between two communicating peers. If this protection can be
circumvented, then it is possible for an attacker to cause harm
without necessarily needing to redirect the return packets.
There are various level of protection in different transport
protocols. For instance, in general TCP packets have to contain a
sequence that falls in the receiver's window to be accepted. If the
TCP initial sequence numbers are random then it is relatively hard
for an off-path attacker to guess the sequence number close enough
for it to belong to the window, and as result be able to inject a
packet into an existing connection. How hard this is depends on the
size of the available window. SCTP provides a stronger mechanism
with the verification tag; an off-path attacker would need to guess
this random 32-bit number. Of course, IPsec provide
cryptographically strong mechanisms which prevent attackers on or off
path to inject packets once the security associations have been
established.
When ingress filtering is deployed between the potential attacker and
the path between the communicating peers, it can prevent the attacker
from using the peer's IP address as source. In that case the packet
injection will fail in today's Internet.
We don't expect a multihoming solution improve the existing degree of
prevention against packet injection. However, it is necessary to
look carefully whether a multihoming solution makes it easier for
attackers to inject packets since the desire to have the peer be
present at multiple locators, and perhaps at a dynamic set of
locators, can potentially result in solutions that, even in the
presence of ingress filtering, make packet injection easier.
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INTERNET-DRAFT Threats to IPv6 multihoming solutions Sept 22, 20043.4. Flooding Attacks Today
In the Internet today there are several ways for an attacker to use a
redirection mechanism to launch DoS attacks that can not easily be
traced to the attacker. An example of this is to use protocols which
cause reflection with or without amplification [PAXSON01].
Reflection without amplification can be accomplished by an attacker
sending a TCP SYN packet to a well-known server with a spoofed source
address; the resulting TCP SYN ACK packet will be sent to the spoofed
source address.
Devices on the path between two communicating entities can also
launch DoS attacks. While such attacks might not be interesting
today, it is necessary to understand them better in order to
determine whether a multihoming solution might enables new types of
DoS attacks.
For example, today if A is communicating with B, then A can try to
overload the path from B to A. If TCP is used A could do this by
sending ACK packets for data that it has not yet received (but it
suspects B has already sent) so that B would send at a rate that
would cause persistent congestion on the path towards A. Such an
attack would seem self-destructive since A would only make its own
corner of the network suffer by overloading the path from the
Internet towards A.
A more interesting case is if A is communicating with B and X is on
the path between A and B, then X might be able to fool B to send
packets towards A at a rate that is faster than A (and the path
between A and X) can handle. For instance, if TCP is used then X can
craft TCP ACK packets claiming to come from A to cause B to use a
congestion window that is large enough to potentially cause
persistent congestion towards A. Furthermore, if X can suppress the
packets from A to B it can also prevent A from sending any explicit
"slow down" packets to B, that is, X can disable any flow or
congestion control mechanism such as Explicit Congestion Notification
[ECN]. Similar attacks can presumably be launched using protocols
that carry streaming media by forging such a protocol's notion of
acknowledgment and feedback.
An attribute of this type of attack is that A will simply think that
B is faulty since its flow and congestion control mechanisms don't
seem to be working. Detecting that the stream of ACK packets is
generated from X and not from A might be challenging, since the rate
of ACK packets might be relatively low. This type of attack might
not be common today because, in the presence of ingress filtering, it
requires that X remain on the path in order to sustain the DoS
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attack. And in the absence of ingress filtering an attacker would
either need to be present on the path initially and then move away,
or the attacker would need to be able to perform the setup of the
communication "blind" i.e., without seeing the return traffic (which
in the case of TCP implies guessing the initial sequence number).
The danger is that the addition of multihoming redirection mechanisms
might potentially remove the constraint that the attacker remain on
the path. And with the current, no-multihoming support, using end-
to-end strong security at a protocol level at (or below) this "ACK"
processing would prevent this type of attack. But if a multihoming
solution is provided underneath IPsec that prevention mechanism would
potentially not exist.
Thus the challenge for multihoming solutions is to not create
additional types of attacks in this area, or make existing types of
attacks significantly easier.
3.5. Address Privacy Today
In today's Internet there is limited ability to track a host as it
uses the Internet because in some cases, such as dialup connectivity,
the host will acquire different IPv4 addresses each time it connects.
However, with increasing use of broadband connectivity, such as DSL
or cable, it is becoming more likely that the host will maintain the
same IPv4 over time. Should a host move around in today's Internet,
for instance, by visiting WiFi hotspots, it will be configured with a
different IPv4 address at each location.
We may also observe that a common practice in IPv4 today is to use
some form of address translation, whether the site is multihomed or
not. This effectively hides the identity of the specific host within
a site; only the site can be identified based on the IP address.
In the cases where it is desirable to maintain connectivity as a host
moves around, whether using layer 2 technology or Mobile IPv4, the
IPv4 address will remain constant during the movement (otherwise the
connections would break). Thus there is somewhat of a choice today
between seamless connectivity during movement and increased address
privacy.
Today when a site is multihomed to multiple ISPs the common setup is
that a single IP address prefix is used with all the ISPs. As a
result it is possible to track that it is the same host that is
communication via all ISPs.
However, when a *host* is multi-homed to several ISP, e.g. through a
GPRS connection and a wireless hot spot, the host is provided with
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different IP addresses on each interface. While the focus of the
multihoming work is on site multihoming, should the solution also be
applicable to host multihoming, the privacy impact needs to be
considered for this case as well.
IPv6 stateless address auto-configuration facilitates IP address
management, but raises some concerns since the Ethernet address is
encoded in the low-order 64 bits of the IPv6 address. This could
potentially be used to track a host as it moves around the network,
using different ISPs etc. IPv6 specifies temporary addresses
[RFC3041] which allow applications to control whether they need
long-lived IPv6 addresses or desire the improved privacy of using
temporary addresses.
Given that there isn't address privacy in site multihoming setups
today, the primary concerns for the "do no harm" criteria are to
ensure that hosts that move around still have the same ability as in
today's Internet to choose between seamless connectivity and improved
address privacy, and also that the introduction of multihoming
support should still provide the same ability as we have in IPv6 with
temporary addresses.
When considering privacy threats it makes sense to distinguish
between attacks make by on-path entities observing the packets flying
by, and attacks by the communicating peer. While it is probably
feasible to prevent on-path entities from correlating the multiple IP
addresses of the host, the fact that the peer needs to be told
multiple IP addresses in order to be able to switch to using
different addresses when communication fails limits the ability of
the host to prevent correlating its multiple addresses. However,
using multiple pseudonyms for a host should be able address this
case.
4. POTENTIAL NEW ATTACKS
This section documents the additional attacks that have been
discovered that result from an architecture where hosts can change
their topological connection to the network in the middle of a
transport session without interruption. This discussion is again
framed in the context where the topological locators may be
independent of the host identifiers used by the transport and
application layer protocols. Some of these attacks may not be
applicable if traditional addresses are used. This section assumes
that each host has multiple locators and that there is some mechanism
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for determining the locators for a correspondent host. We do not
assume anything about the properties of these mechanisms. Instead,
this list will serve to help us derive the properties of these
mechanisms that will be necessary to prevent these redirection
attacks.
Depending on the purpose of the redirection attack we separate the
attacks into several different types.
4.1. Cause Packets to be Sent to the Attacker
An attacker might want to receive the flow of packets, for instance
to be able to inspect and/or modify the payload or to be able to
apply cryptographic analysis to cryptographically protected payload,
using redirection attacks.
4.1.1. Once Packets are Flowing
This might be viewed as the "classic" redirection attack.
While A and B are communicating X might send packets to B and claim:
"Hi, I'm A, send my packets to my new location." where the location
is really X's location.
"Standard" solutions to this include requiring the host requesting
redirection somehow be verified to be the same host as the initial
host to establish communication. However, the burdens of such
verification must not be onerous, or the redirection requests
themselves can be used as a DoS attack.
To prevent this type of attack, a solution would need some mechanism
that B can use to verify whether a locator belongs to A before B
starts using that locator, and be able to do this when multiple
locators are assigned to A.
4.1.2. Time-shifting Attack
The term "time-shifting attack" is used to describe an attackers
ability to perform an attack after no longer being on the path. Thus
the attacker would have been on the path at some point in time,
snooping and/or modifying packets, and later when the attacker is no
longer on the path, it launches the attack.
In the current Internet, it is not possible to perform such attacks
to redirect packets. But for sometime after moving away the attacker
can cause a DoS attack e.g. by leaving a bogus ARP entry in the nodes
on the path or by forging TCP Reset packets based on having seen the
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TCP Initial Sequence Numbers when it was on the path.
It would be reasonable to require that a multihoming solution limit
the ability to redirect and/or DoS traffic to a few minutes after the
attacker has moved off the path.
4.1.3. Premeditated Redirection
This is a variant of the above where the attacker "installs" itself
before communication starts.
For example, if the attacker X can predict that A and B will
communicate in the (near) future, then the attacker can tell B: "Hi,
I'm A and I'm at this location". When A later tries to communicate
with B, will B believe it is really A?
If the solution to the classic redirection attack is based on "prove
you are the same as initially", then A will fail to prove this to B
since X initiated communication.
Depending on details that would be specific to a proposed solution,
this type of attack could either cause redirection (so that the
packets intended for A will be sent to X) or they could cause DoS
(where A would fail to communicate with B since it can't prove it is
the same host as X).
To prevent this attack, the verification whether a locator belongs to
the peer can not simply be based on the first peer that made contact.
4.1.4. Using Replay Attacks
While the multihoming problem doesn't inherently imply any
topological movement it is useful to also consider the impact of site
renumbering in combination with multihoming. In that case the set of
locators for a host will change each time its site renumbers and at
some point in time after a renumbering event the old locator prefix
might be reassigned to some other site.
This potentially opens up the ability for an attacker to replay
whatever protocol mechanism was used to inform a host of a peer's
locators so that the host would incorrectly be lead to believe that
the old locator (set) should be used even long after a renumbering
event. This is similar to the risk of replay of Binding Updates in
[MIPv6] but the time constant is quite different; Mobile IPv6 might
see movements every second while site renumbering followed by
reassignment of the site locator prefix might be a matter of weeks or
months.
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To prevent such replay attacks the protocol which is used to verify
which locators can be used with a particular identifier needs some
replay protection mechanism.
Also, in this space one needs to be concerned about potential
interaction between such replay protection and the administrative act
of reassignment of a locator. If the identifier and locator
relationship is distributed across the network one would need to make
sure that the old information has been completely purged from the
network before any reassignment. Note that this does not require an
explicit mechanism. This can instead be implemented by locator reuse
policy and careful timeouts of locator information.
4.2. Cause Packets to be Sent to a Black Hole
This is also a variant of the classic redirection attack. The
difference is that the new location is a locator that is nonexistent
or unreachable. Thus the effect is that sending packets to the new
locator causes the packets to be dropped by the network somewhere.
One would expect that solutions which prevent the previous
redirection attacks would prevent this attack as a side effect, but
it makes sense to include this attack here for completeness.
Mechanisms that prevented a redirection attack to the attacker should
also prevent redirection to a black hole.
4.3. Third Party Denial-of-Service Attacks
An attacker can use the ability to perform redirection to cause
overload on an unrelated third party. For instance, if A and B are
communicating then the attacker X might be able to convince A to send
the packets intended for B to some third node C. While this might
seem harmless at first, since X could just flood C with packets
directly, there are a few aspects of these attacks that cause
concern.
The first is that the attacker might be able to completely hide its
identity and location. It might suffice for X to send and receive a
few packets to A in order to perform the redirection, and A might not
retain any state on who asked for the redirection to C's location.
Even if A had retained such state, that state would probably not be
easily available to C, thus C can't determine who was the attacker
once C is being DoS'ed.
The second concern is that with a direct DoS attack from X to C, the
attacker is limited by the bandwidth of its own path towards C. If
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the attacker can fool another host like A to redirect its traffic to
C then the bandwidth is limited by the path from A towards C. If A
is a high-capacity Internet service and X has slow (e.g., dialup)
connectivity this difference could be substantial. Thus in effect
this could be similar to packet amplifying reflectors in [PAXSON01].
The third, and final concern, is that if an attacker only need a few
packets to convince one host to flood a third party, then it wouldn't
be hard for the attacker to convince lots of hosts to flood the same
third party. Thus this could be used for Distributed Denial-of-
Service attacks.
In today's Internet the ability to perform this type of attack is
quite limited. In order for the attacker to initiate communication
it will in most cases need to be able to receive some packets from
the peer (the potential exception being combining this with TCP
sequence number guessing type of techniques). Furthermore, to the
extent that parts of the Internet uses ingress filtering [INGRESS],
even if the communication could be initiated it wouldn't be possible
to sustain it by sending ACK packets with spoofed source addresses
from an off-path attacker.
If this type of attack can't be prevented there might be mitigation
techniques that can be employed. For instance, in the case of TCP it
would help if TCP slow-start was triggered when the destination
locator changes. (Folks might argue that, separately from security,
this would be the correct action for congestion control since TCP
might not have any congestion-relation information about the new path
implied by the new locator). Applying this technique to other
transport protocols which perform different forms of (TCP friendly)
congestion control might be more difficult since the lower layers
generally lack an API to provide such information to the transports.
Also, for other protocols, this might be less beneficial, since other
transports might not adapt rapidly and could view the suggestion of
congestion as being more severe than a simple deficit of congestion
information.
4.3.1. Basic Third Party DoS
Assume that X is on a slow link anywhere in the Internet. B is on a
fast link (gigabits; e.g., a media server) and A is the victim.
X could flood A directly but is limited by its low bandwidth. If X
can establish communication with B, ask B to send it a high-speed
media stream, then X can presumably fake out the
"acknowledgments/feedback" needed for B to blast out packets at full
speed. So far this only hurts X - and the path between X and the
Internet. But if X could also tell B "I'm at A's locator" then X has
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effectively used this redirection capability in multihoming to
amplify its DoS capability, which would be a source of concern.
One could envision rather simple techniques to prevent such attacks.
For instance, before sending to a new peer locator perform a clear
text exchange with the claimed new locator of the form "Are you X?"
resulting in "Yes, I'm X.". This would suffice for the simplest of
attacks. However, as we will see below, more sophisticated attacks
are possible.
4.3.2. Third Party DoS with On-Path Help
The scenario is as above but in addition the attacker X has a friend
Y on the path between A and B:
----- ----- -----
| A |--------| Y |--------| B |
----- ----- -----
/
/
/
/
/
/
-----
| X |
-----
With the simple solution suggested in the previous section, all Y
might need to do is to fake a response to the "Are you X?" packet,
and after that point in time Y might not be needed; X could
potentially sustain the data flow towards A by generating the ACK
packets. Thus it would be even harder to detect the existence of Y.
Furthermore, if X is not the actual end system but an attacker
between some node C and B, then X can claim to be C, and no finger
can be pointed at X either:
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----- ----- -----
| A |--------| Y |--------| B |
----- ----- -----
/
/
/
/
/
/
----- -----
| C |-------| X |
----- -----
Thus with two attackers on different paths, there might be no trace
of who did the redirection to the 3rd party once the redirection has
taken place.
A specific case of this is when X=Y, and X is located on the same LAN
as B.
A potential way to make such attacks harder would be to use the last
received (and verified) source locator as the destination locator.
That way when X sends the ACK packets (whether it claims to be X or
C) the result would be that the packet flow from B would switch back
towards X/C, which would result in an attack similar to what can be
performed in today's Internet.
Another way to make such attacks harder would be to perform periodic
verifications that the peer is available at the locator, instead of
just one when the new locator is received.
A third way that a multihoming solution might address this is to
ensure that B will only accept locators that can be authenticated to
be synonymous with the original correspondent. It must be possible
to securely ensure that these locators form an equivalence class. So
in the first example, not only does X need to assert that it is A,
but A needs to assert that it is X.
4.4. Accepting Packets from Unknown Locators
The multihoming solution space does not only affect the destination
of packets; it also raises the question from which sources packets
should be accepted. It is possible to build a multihoming solution
that allows traffic to be recognized as coming from the same peer
even if there is a previously unknown locator present in the source
address field. The question is whether we want to allow packets from
unverified sources to be passed on to transport and application layer
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protocols.
In the current Internet, an attacker can't inject packets with
arbitrary source addresses into a session if there is ingress
filtering present, so allowing packets with unverified sources in a
multihoming solution would fail our "no worse than what we have now"
litmus test. However, given that ingress filtering deployment is far
from universal and ingress filtering typically wouldn't prevent
spoofing of addresses in the same subnet, requiring rejecting packets
from unverified locators might be too stringent.
An example of the current state are the ability to inject RST packets
into existing TCP connections. When there is no ingress filtering in
the network, this is something that the TCP endpoints need to sort
out themselves. However, deploying ingress filtering helps in
today's Internet since an attacker is limited in the set of source
address it can use.
A factor to take into account to determine the "requirement level"
for this is that when IPsec is used on top of the multihoming
solution, then IPsec will reject such spoofed packets. (Note that
this is different than in the redirection attack cases where even
with IPsec an attacker could potentially cause a DoS attack.)
There might also be a middle ground where arbitrary attackers are
prevented from injecting packets by using the SCTP verification tag
type of approach [SCTP]. (This is a clear-text tag which is sent to
the peer which the peer is expected to include in each subsequent
packet.) Such an approach doesn't prevent packet injection from on-
path attackers (since they can observe the verification tag), but
neither does ingress filtering.
4.5. New Privacy Considerations
While introducing identifiers can be helpful by providing ways to
identify hosts across events when its IP address(es) might change,
there is a risk that such mechanisms can be abused to track the
identity of the host over long periods of time, whether using the
same (set of) ISP(s) or moving between different network attachment
points. Designers of solutions to multihoming need to be aware of
this concern.
Introducing the multihoming capability inherently implies that the
communicating peers need to know multiple locators for each other in
order to be resilient to failures of some paths/locators. In
addition, if the multihoming signaling protocol doesn't provide
privacy it might be possible for 3rd parties on the path to discover
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many or all the locators assigned to a host, which would increase the
privacy exposure compared to a multihomed host today.
A solution could address this for instance by allowing each host to
have multiple identifiers at the same time and perhaps even changing
the set of identifiers that are used over time. Such an approach
could be analogous to what is done for IPv6 addresses in [RFC3041].
5. GRANULARITY OF REDIRECTION
Different multihoming solutions might approach the problem at
different layers in the protocol stack. For instance, there have
been proposals for a shim layer inside IP as well as transport layer
approaches. The former would have the capability to redirect an IP
address while the latter might be constrained to only redirect a
single transport connection. This difference might be important when
it comes to understanding the security impact.
For instance, premeditated attacks might have quite different impact
in the two cases. In an IP-based multihoming solution a successful
premeditated redirection could be due to the attacker connecting to a
server and claiming to be 'A' which would result in the server
retaining some state about 'A' which it received from the attacker.
Later, when the real 'A' tries to connect to the server, the
existence of this state might mean that 'A' fails to communicate, or
that its packets are sent to the attacker. But if the same scenario
is applied to a transport-layer approach then the state created due
to the attacker would perhaps be limited to the existing transport
connection. Thus while this might prevent the real 'A' from
connecting to the server while the attacker is connected (if they
happen to use the same transport port number) most likely it would
not affect 'A's ability to connect after the attacker has
disconnected.
A particular aspect of the granularity question is the direction
question; will the created state be used for communication in the
reverse direction from the direction when it was created? For
instance, if the attacker 'X' suspects that 'A' will connect to 'B'
in the near future, can X connect to A and claim to be B and have
that later make A connect to the attacker instead of to the real B?
Note that transport layer approaches are limited to the set of
"transport" protocols that the implementation makes aware of
multihoming. In many cases there would be "transport" protocols that
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are unknown to the multihoming capability of the system, such as
applications built on top of UDP. To understand the impact of the
granularity question on the security, one would also need to
understand how such applications/protocols would be handled.
A property of transport granularity is that the amount of work
performed by a legitimate host is proportional to the number of
transport connections it creates that uses the multihoming support,
since each such connection would require some multihoming signaling.
And the same is true for the attacker. This means that an attacker
could presumably do a premeditated attack for all TCP connections to
port 80 from A to B, by setting up 65,536 (for all TCP source port
numbers) to the server B and causing B to think those connections
should be directed to the attacker and keeping those TCP connections
open. Any attempt to make legitimate communication more efficient,
e.g., by being able to signal for multiple transport connections at a
time, would provide as much relative benefit for an attacker as the
legitimate hosts.
Discussion: Perhaps the key issue is not about the granularity,
but about the lifetime of the state that is created? In a
transport-layer approach the multihoming state would presumably be
destroyed when the transport state is deleted as part of closing
the connection. But an IP-layer approach would have to rely on
some timeout or garbage collection mechanisms perhaps combined
with some new explicit signaling to remove the multihoming state.
The coupling between the connection state and multihoming state in
the transport-layer approach might make it more expensive for the
attacker, since it needs to keep the connections open. Is this
the case?
In summary, there are issues we don't yet understand well about
granularity and reuse of the multihoming state.
6. MOVEMENT IMPLICATIONS?
In the case when nothing moves around we have a reasonable
understanding of the security requirements. Something that is on the
path can be a MiTM in today's Internet and a multihoming solution
doesn't need to make that aspect any more secure.
But it is more difficult to understand the requirements when hosts
are moving around. For instance, a host might be on the path for a
short moment in time by driving by an 802.11 hotspot. Would we or
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would we not be concerned if such a drive-by (which many call a
"time-shifting" attack) would result in the temporarily on-path host
being able to act as a MiTM for future communication? Depending on
the solution this might be possible by the attacker causing
multihoming state to be created in various peer hosts while the
attacker was on the path, and that state remaining in the peers for
some time.
The answer to this question doesn't seem to be obvious even in the
absence of any new multihoming support. We don't have much
experience with hosts moving around that are able to attack things as
they move. In Mobile IPv6 [MIPv6] a conservative approach was taken
which limits the effect of such drive-by attacks to the maximum
lifetime of the binding, which is set to a few minutes.
With multihoming support the issue gets a bit more complicated
because we explicitly want to allow a host to be present at multiple
locators at the same time, thus there might be a need to distinguish
between the host moving between different locators, and the host
sending packets with different source locators because it is present
at multiple locators without any topological movement.
Note that the multihoming solutions that have been discussed range
from such drive-by's being impossible (for instance, due to a strong
binding to a separate identifier as in HIP, or due to reliance on the
relative security of the DNS for forward plus reverse lookups in
NOID), to systems that are first-come/first-serve (WIMP being an
example with a separate ID space, a MAST approach with a PBK being an
example without a separate ID space) that allow the first host which
is using an ID/address to claim that without any time limit.
7. OTHER SECURITY CONCERNS
The protocol mechanisms added as part of a multihoming solution
shouldn't introduce any new DoS in the mechanisms themselves. In
particular, care must be taken not to:
- create state on the first packet in an exchange, since that could
result in state consumption attacks similar to the TCP SYN
flooding attack.
- perform much work on the first packet in an exchange (such as
expensive verification)
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There is a potential chicken-and-egg problem here, because
potentially one would want to avoid doing work or creating state
until the peer has been verified, but verification will probably need
some state and some work to be done.
A possible approach that solutions might investigate is to defer
verification until there appears to be two different hosts (or two
different locators for the same host) that want to use the same
identifier.
Another possible approach is to first establish communications, and
then perform verification in parallel with normal data transfers.
Redirection would only be permitted after verification was complete,
but prior to that event, data could transfer in a normal, non-
multihomed manner.
Finally, the new protocol mechanisms should be protected against
spoofed packets, at least from off-path sources, and replayed
packets.
8. SECURITY CONSIDERATIONS
In section 3 we discussed existing protocol-based redirection
attacks. But there are also non-protocol redirection attacks. An
attacker which can gain physical access to one of
- The copper/fiber somewhere in the path.
- A router or L2 device in the path.
- One of the end systems
can also redirect packets. This could be possible for instance by
physical break-ins or by bribing staff that have access to the
physical infrastructure. Such attacks are out of scope for this
discussion, but are worth to keep in mind when looking at the cost
for an attacker to exploit any protocol-based attacks against
multihoming solutions; making protocol-based attacks much more
expensive to launch than break-ins/bribery type of attacks might be
overkill.
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The following changes have been made since draft-nordmark-multi6-threats-02:
o Editorial clarifications based on WG comments.
o Removed the ULP term and it's usage since it was potentially
confusing.
o Added a current state of privacy as the basis for "do no harm"
o Added some background information about authentication,
authorization, and identifier ownership.
o Improvements to Appendix B
The following changes have been made since draft-nordmark-multi6-threats-01:
o Editorial clarifications based on comments from Brian.
The following changes have been made since draft-nordmark-multi6-threats-00:
o Added reference to [DNS-THREATS] and clarified that attackers on
the path between the host and the DNS servers can redirect traffic
today.
o Added a section on existing packet injection attacks to talk about
TCP sequence number guessing etc.
o Clarified ingress filtering relationship in section on today's
flooding attacks.
o Added a new section on granularity to list some issues about
transport-level versus IP-level approaches and what we understand
about the differences in security. This is still very much a work
in progress.
o Added a new section on movement to discuss how things change when
hosts move around the network. This is still very much a work in
progress.
o Added Appendix B - but this should probably be moved to a
different document to keep this document focused on the threats.
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APPENDIX B: SOME SECURITY ANALYSIS
When looking at the proposals that have been made for multihoming
solutions and the above threats it seems like there are two separable
aspects of handling the redirection threats:
- Redirection of existing communication
- Redirection of an identity before any communication
This seems to be related to the fact that there are two different
classes of use of identifiers. One use is for:
o Initially establishing communication; looking up a FQDN to find
something which is passed to a connect() or sendto() API call.
o Comparing whether a peer entity is the same peer entity as in some
previous communication.
o Using the identity of the peer for future communication
("callbacks") in the reverse direction, or to refer to a 3rd party
("referrals").
The other use of identifiers is as part of being able to redirect
existing communication to use a different locator.
The first class of use seems to be related to something about the
ownership of the identifier; proving the "ownership" of the
identifier should be sufficient in order to be authorized to control
which locators are used to reach the identifier.
The second class of use seems to be related to something more
ephemeral. It seems to be sufficient to be able to prove that the
entity is the same as the one that initiated the communication to be
able to redirect the existing communication to some other locator.
One can view this as proving the ownership of some context, where the
context is established around the time when the communication is
initiated.
Preventing unauthorized redirection of existing communication can be
addressed by a large number of approaches which are based on setting
up some form of security material at the beginning of communication,
and later using the existence of that material for one end to prove
to the other that it remains the same. An example of this is Purpose
Built Keys [PBK]. One can envision different approaches for such
schemes with different complexity, performance, and resulting
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security such as anonymous Diffie-Hellman exchange, the reverse hash
chains presented in [WIMP], or even a clear-text token exchanged at
the initial communication.
However, the mechanisms for preventing unauthorized use of an
identifier can be quite different. One way to prevent premeditated
redirection is to simply not introduce a new identifier name space
but instead rely on existing name space(s), a trusted third party,
and a sufficiently secure way to access the third party, as in
[NOID]. Such an approach relies on the third party (DNS in the case
of NOID) as the foundation. In terms of multihoming state creation,
in this case premeditated redirection is as easy or as hard as
redirecting an IP address today. Essentially this relies on the
return-routability check associated with a roundtrip of communication
which verifies that the routing system delivers packets to the IP
address in question.
Alternatively, one can use the crypto-based identifiers such as in
[HIP] or crypto-generated addresses as in [CBHI], which both rely on
public-key crypto, to prevent premeditated attacks. In some cases it
is also possible to avoid the problem by having (one end of the
communication) use ephemeral identifiers as the initiator does in
[WIMP]. This avoids premeditated redirection by detecting that some
other entity is using the same identifier at the peer and switching
to use another ephemeral ID. While the ephemeral identifiers might
be problematic when used by applications, for instance due to
callbacks or referrals, it is an interesting observation that for the
end that has the ephemeral identifier, one can skirt around the
premeditated attacks (assuming the solution has a robust way to pick
a new identifier when one is in use/stolen).
Assuming the problem can't be skirted around by using ephemeral
identifiers there seem to be 3 types of approaches which can be used
to establish some form of identity ownership:
- A trusted third party, which states that a given identity is
reachable at a given set of locators, so the endpoint reached at
one of this locators is the owner of the identity.
- Crypto-based identifiers or crypto-generated addresses where the
public/private key pair can be used to prove that the identifier
was generated by the node knowing the private key (or by another
node whose public key hashes to the same value)
- A static binding, as currently defined in IP, where you trust that
the routing system will deliver the packets to the owner of the
locator, and since the locator and the identity are one, you prove
identity ownership as a sub-product.
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A solution would need to combine elements which provide protection
against both premeditated and on-going communication redirection.
This can be done in several ways, and the current set of proposals do
not appear to contain all useful combinations. For instance, the HIP
CBID property could be used to prevent premeditated attacks while the
WIMP hash chains could be used to prevent on-going redirection. And
there are probably other interesting combinations.
A related, but perhaps separate aspect, is whether the solution
provides for protection against Man-in-The-Middle attacks with on-
path attackers. Some schemes, such as [HIP] and [NOID] do, but given
that an on-path attacker can see and modify the data traffic whether
or not it can modify the multihoming signaling, this level of
protection seems like overkill. Protecting against on-path MiTM for
the data traffic can be done separately using IPsec, TLS, etc.
Finally, preventing third party DoS attacks is conceptually simpler;
it would suffice to somehow verify that the peer is indeed reachable
at the new locator before sending a large number of packets to that
locator.
Just as the redirection attacks are conceptually prevented by proving
at some level the ownership of the identifier or the ownership of the
communication context, third party DoS attacks are conceptually
prevented by showing that the endpoint is authorized to use a given
locator.
The currently known approaches for showing such authorization are:
- Return routability, that is, if an endpoint is capable of
receiving packets at a given locator, it is because he is
authorized to do so. This relies to routing being reasonably hard
to subvert to deliver packets to the wrong place. While this
might be the case when routing protocols are used with reasonable
security mechanisms and practices, it isn't the case on a single
link where ARP and Neighbor Discovery can be easily spoofed.
- Third trusted party. A third party establishes that a given
identity is authorized to use a given set of locators (for
instance the DNS)
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